feat(chat): read-only Chat sidebar tab (Phase 3)

Add the first user-visible HushChat surface: a Chat tab in the sidebar with a
two-pane, read-only conversation view — a conversation list on the left and the
selected thread on the right — reading from the App-owned ChatService store.

- New src/ui/windows/chat_tab.{h,cpp} (RenderChatTab(App*), mirroring the
  Contacts tab). Conversations are sorted by most-recent activity; peer
  z-addresses resolve to contact names via the address book when known; each
  message shows sender + timestamp + wrapped body, with a contact-request tag
  and a read-only footer (composing arrives in a later phase). Empty states
  cover "wallet locked / identity not ready" and "no conversations yet".
- Sidebar wiring: NavPage::Chat + registry entry (static_assert stays balanced),
  NavPageSurface + GetNavIconMD (ICON_MD_CHAT), dispatch in app.cpp, trace/sweep
  page names, and App::chatService() accessor. The tab is gated on the new
  WalletUiSurface::Chat, which returns DRAGONX_ENABLE_CHAT != 0 — so it only
  appears in chat-enabled builds and is hidden (and unreachable) by default.
- i18n: English defaults for the chat nav label + hint strings.

Verified: Linux + Windows(mingw) build with chat ON, ctest 100%, hygiene clean;
caches restored to the OFF default. The screenshot sweep now includes the Chat
tab (sweepPageName "chat") for per-theme visual review.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/wallet/wallet_capabilities.h

@@ -16,6 +16,10 @@
 #define DRAGONX_ENABLE_LITE_BACKEND 0
 #endif
 
+#ifndef DRAGONX_ENABLE_CHAT
+#define DRAGONX_ENABLE_CHAT 0
+#endif
+
 namespace dragonx {
 namespace wallet {
 
@@ -35,6 +39,7 @@ enum class WalletUiSurface {
     Receive,
     History,
     Contacts,   // address book / chat roster; available in both variants (default: return true)
+    Chat,       // HushChat conversations; gated on DRAGONX_ENABLE_CHAT (default OFF)
     Mining,
     Market,
     Console,
@@ -159,6 +164,8 @@ constexpr bool isUiSurfaceAvailable(const WalletCapabilities& capabilities,
         case WalletUiSurface::LiteNetwork:
         case WalletUiSurface::LiteConsole:
             return !capabilities.fullNodePagesAvailable;  // lite builds only
+        case WalletUiSurface::Chat:
+            return DRAGONX_ENABLE_CHAT != 0;  // experimental; compiled-in only when the feature is on
         case WalletUiSurface::BootstrapDownload:
         case WalletUiSurface::SetupWizard:
         case WalletUiSurface::NodeSettings: